In the electronic components semiconductor industry it is generally required to subject prototypes and production samples of semiconductor devices (chips or modules) to thorough electrical testing. Since specifications of a device typically include the range of ambient temperatures over which it should be operable, each device under test (DUT), and more specifically its casing, must generally be held during part of such testing at each of the extreme temperature values of the specified range (thus simulating the required extreme ambient temperature values). Such extreme values are typically between 125 and 165 degrees centigrade, at the high end, and between −40 and −70 degrees centigrade, at the low end. The process of thus keeping the case temperature of a DUT at one or the other of the specified extreme values is known as temperature forcing and is achieved, in common practice, by placing a heat conducting device in tight thermal contact with the DUT's casing and controlling its temperature so as to be held near the desired value. The heat conducting device and the system that controls its temperature are together referred to as a temperature forcing system (TFS).
Moreover, as is well known, operation of a semiconductor chip or any other electronic component is an exothermic process, wherein electric power fed to it is converted to heat, thereby tending to raise the temperature of the chip. This heat must be dissipated by the ambient air and/or external devices, as well as, generally, by the temperature forcing system, in order to limit the rise of the temperature, leaving the latter in equilibrium at the desire level. This is particularly true when testing at the low temperature range. When testing at a high extreme temperature, however, the exothermic process of the DUT may at times be insufficient to raise its casing temperature to the desired level, as all of the generated heat is dissipated by the ambient air and external devices. In this case the temperature forcing system, rather than dissipating heat, must supply heat to the DUT through its casing.
Two important requirements govern such temperature forcing: One requirement is that the temperature of the DUT, or its casing, be monitored and held at the desired level quite accurately and constant (say—within 0.1 degrees C.). The other requirement is that the controlled temperature be switchable between the two extreme values (or to any other values) within a relatively short time (say—at a rate of 10-60 degrees C. per minute). It is noted that the temperature must be held constant even while varying operations in the chip (per test procedures) cause varying amounts of heat to be generated therein; the present invention aims at holding the DUT's temperature constant even while the input power dissipated by the DUT varies between a fraction of a watt and several hundred watts. Another, obvious, requirement is that any test setup be operational with a wide variety of chip types to be tested, having different heat-generating characteristics.
During testing, a semiconductor device (e.g. a packaged chip or an electronic module) is typically held in a test jig so that electrical terminals on its bottom surface are in contact with appropriately configured electrical test circuitry, while its top surface is clear for temperature forcing. Other test configurations are also possible and are equally addressed by the present invention.
A typical temperature forcing system of prior art comprises a thermal head that is placeable in thermal contact with the DUT, a chiller and a circulation system that circulates a heat transfer fluid between two heat exchangers—one in the chiller and one in the thermal head. The chiller is a conventional refrigeration system, operational to extract heat from its heat exchanger and thus to cool the heat transfer fluid to substantially below the extreme low temperature of the desired testing range. The latter is generally designed to remain liquid throughout the circulation system and over the entire range of the testing temperatures. When passing through the thermal head's heat exchanger, the transfer fluid extracts heat from the thermal head, thus, in turn, cooling it.
Such a prior-art system has three major drawbacks:    (a) The presence of two heat exchangers in tandem causes a relatively large cumulative amount of temperature differential between the chiller and the DUT, thus reducing the efficiency of the process and placing a sometimes unacceptable limit on how low the temperature of the latter may be forced with a simple (single-stage) refrigeration system.    (b) Heat dissipation in the thermal head's heat exchanger is based on the principle of Fluid Forced Heat convection, whose heat transmission factors are low (by several orders of magnitude relative to the principle on which the present invention is based); this seriously limits the rate at which the DUT temperature may be changed during testing. The relatively large heat capacity of the circulating transfer fluid further limits the rate at which the temperature may be switched.